Thermally hardening steel



1933- E. c. BAIN Er AL THERMALLY HARDENING STEEL Filed Nov. 20, 1931 4 he tsheet 2 141/5 TE NI TE m m m w W m ab MQEEQQSE T 4 a m w m w B .3 m L m m .m N NM 0 omopw & M MHM w cMsps -5 TRANSFORMED 7 TRANSFORMED TRANSFORMATION ENDS 4 m 25 28 Q3? M l1 m m $23 11 m m w m a 8:? o H E I m T 8& M -w T m m m w F 87 M m 1 m S m 15w 0 a 5 y Ed INVENTORS BY wq ATTORN EYS E. C. BAIN El" AL THERMALLY HARDENING STEEL Filed Nov. 20, 1931 4 Sheets-Sheet 5 Roqkwefl. HIARDNESS 0F PRODUCT am zz art ATTORNEYS INVENTO RS Edgar CB Edmzg zdfibazj'f STEELE Aug. 29, 1933.

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Patented Aug. 29, 1933 UNITED STA THERMALLY HARDENIN G STEEL Edgar C. Bain, Short Hills, and Edmund S. lDavenport, Newark, N. J., assignors to United States Steel Corporation, New York, N. Y., a Corporation of New Jersey Application November 20, 1931 Serial No. 576,398

11 Claims.

This invention relates to metallurgy and more particularly to the thermal hardening of steels, alloys and the like in which the hardening is obtained by reason of the phenomenon of compound particle solution and subsequent precipitation in the metal at the various appropriate temperatures. More specifically the present invention relates to all carbon and'to all alloy steels in which the carbon content thereof may vary in specific amounts up to values as high as commonly employed, for example 2% or 3%, and in which the alloying elements are not present in amounts sufficient to inhibit the phenomenon of austenite formation at elevated temperatures nor to prevent the conversion, at lower temperatures, of the austenite thus formed into the well known crystal forms of pearlite, sorbite, troostite, martensite or the like.

One of the objects of the present invention is to provide a new method of thermally hardening carbon and alloy steels and the like.

Another object of the present invention is to provide an improved thermally hardened carbon or alloy steel product.

Still another object of the present invention is to adapt the novel thermal hardening method of the present invention to the hardening of fabricated products such as steel wire, rope, rods, sheet, plate, cutlery, tools, dies. ball-bearings, machine parts, railroad rails and the like.

Other objects and advantages will be apparent as the invention is more fully disclosed.

In accordance with the objects of the present invention we have discovered that by the'proper control of the transformation of austenite to crystal structures stable at temperatures below the so-called transformation temperature of steel, but above the temperature of so-called martensite formation a hardened steel product can be obtained which is superior to the hardened steel products produced by the heretofore employed quench hardening methods wherein martensite is utilized as a hardening constituent.

When a carbon or alloy steel mass is quenched from a temperature above its transformation temperature range, at which temperature the steel is in the so-called austenitic form, to a temperature below this range, the rate of conversion of the austenite to pearlite, sorbite, troostite or martensite crystal'structures which are formed at the lower temperature varies markedly. In general it may be said' that there are two zones of rapid austenite conversion. With carbon or low alloy steels one conversion zone lies at a temperature approximating 1200 F. to 900 F.

at which the relatively soft pearlite structure is formed. The second zone lies in the temperature range approximating 300 F. or below, wherein rapid conversion of austenite to martensite is obtained.

Between these temperature zones, the rate of conversion of austenite to sorbite, troostite or to admixtures of these with fine pearlite, is relatively slow, and, in general, may be said to decrease with decrease in temperatures from temperatures approximating 1000 F. to temperatures approximating 400 F. Below this temperature range the rate of conversion to martensite becomes rapidly accelerated to become, at temperatures approximating 200 F. andbelow, a matter only of seconds, rather than of minutes orof hours.

The specific temperatures which compass the two zones of rapid conversion of austenite to pearlite and of austenite to martensite are found, in any given steel alloy, to depend in part upon the carbon content and also upon the specific alloy constitutents present. The specific rates of conversion of the austenite at the intermediate temperatures to the troostite and fine pearlite (or sorbite) admixtures also markedly vary with variations in the carbon and specific alloy content of the steel alloy.

It can be generally stated, however, that after the steel has been rapidly quenched through the temperature zone at which rapid pearlite conversion is obtained and the quenching terminated at a temperature above that of rapid martensite conversion, it thereafter may be maintained at this intermediate temperature for relatively prolonged intervals of time before total conversion of 90 the austenite to the specific crystal structure characteristic of that temperature is obtained, and that the specific time interval required increases with decreased temperatures down to about 300 F. In general the lower the tempera- 95 tureof the transformation the harder the product of the transformation, ranging from pearlite 10-15 Rockwell C to martensite 6368 Rockwell C.

We have discovered that a steel product hardened by the practice of the present invention has improved physical properties as indicated by a higher impact strength, increased ductility, etc. than steel products of the same composition hardened to the same degree by prior art quench and temper methods.

This increase in impact strength is due we believe to the fact that during the quench to these low temperatures the conversion of austenite to martensite is accompanied by certain volume changes which set up severe internal stresses and strains in the metal body thereby developing cracks and fissures. Occasionally these may only be detected microscopically.

By the practice of the present invention the development of these cracks and fissures is substantially eliminated and the thermally hardened metal body is thereby greatly improved in ductility and impact strength.

We have adapted the novel thermal hardening process of the present invention to various fabricated articles comprised of carbon or of alloy steels such as wire, cable, sheet, strip, rods, steel rails, beams, plates, cutlery, tools, dies, ballbearings, machine parts and the like. As a specific embodiment of the practice of the present invention we will disclose the method we have devised in the heat treatment of wire.

Before specifically disclosing the same reference should be made to the following drawings wherein:

Fig. 1 is a schematic view of apparatus adapted for the thermal hardening of wire, rope or cable, strip metal and the like;

Fig. 2 is a chart illustrating the thermal hardening time curve for one type of steel; and

Fig. 3 is a chart illustrating the relative hardness of six common steel alloys when hardened at various constant temperatures in accordance with the present invention. Curve B represents the steel of Fig. 2.

Fig. 4 is a chart showing the marked improvement in the impact strength for equivalent hardnesses in a steel treated by the present invention over the same steel quenched and drawn by prior art methods.

Referring to Fig. 1, the apparatus disclosed therein comprises a preheating furnace 1 and a hardening bath 2 to quench to temperatures be.- low the temperature of rapid pearlite formation the wire cable or strip as it emerges from furnace 1, the precise quenching temperature being determined by the desired hardness and the specific steel alloy composition. Wire 5 (cable or strip) is carried on unwinding reel 6, through preheating furnace 1 and hardening bath 2 to winding reel 7 in any convenient manner substantially as shown. Means (not shown) are provided to wind the wire upon reel 7 at a controllably variable rate. Thermostat contol means 8 and 9 are provided for furnace 1 and bath 2 to maintain predetermined desired temperatures therein. Preferably, electrical heating means are employed for furnace 1 and bath 2 as indicated but other heating means may be employed, if desired.

If desired, an inert or reducing atmosphere may be provided in furnace 1 or over bath 2 to prevent undue surface oxidation or corrosion of the wire, cable or strip being conditioned.

If desired, additional quenching means may be provided intermediate furnace 1 and quenching bath 2. Some steel compositions may require only a slight temperature reduction and an air blast cooling may be used. Other steel compositions may require an oil or water quench for which other arr gements may be made. Water may be spraye upon the surface of the wire or the wire may be caused to dip for a brief interval of time below the surface of a water or oil bath. With strip or sheet material we may use water cooled rolls to obtain the desired quenching action. In general, and particularly with sections this additional quenching means may be omitted.

In some instances it may be desirable to obtain moderate hardness by permitting a part of the austenite to transform to martensite by quenching after having permitted the bulk of or a desired proportion of the same to transform over to pearlite, sorbite or troostite crystal forms in the hardening bath 2. Therefore by proper adjustment of the temperatures and the rate of travel of the wire, cable or strip 5 this additional quenching may be obtained by providing an air blast, water or oil bath between quenching bath 2 and winding reel '7. Where the time interval in bath 2 is sufficient to obtain entire conversion of the austenite to the crystal forms other than martensite, martensite then will not be formed when and if the wire 5 is quenched from the hardening bath temperatures before passing to the winding reel 7.

Referring to the hardening bath 2, the material 10 comprising the same is preferably comprised of lead or a lead alloy adapted to give the desired fluidity at the predetermined hardening temperature, and container 11 may be comprised of any suitable material non-reactive therewith. Electrical heating elements 12 also are subject to wide variation in type and manner of control without departing essentially from the nature and scope of the present invention. Means 13 adapted to submerge the wire 5 below the surface of the bath 10 may be comprised of the common rotatable pulley type indicated or may be any other convenient means as desired. Bath 2 may be sealed from the atmosphere or open thereto as desired, and a superposed inert or a reducing gas may be employed, if desired.

A steel wire of the composition indicated in Figs. 2 and 4 heat treated to develop a hardness of 50 Rockwell C by the present invention developed a tensile strength of 283,000 pounds per sq. in. accompanied by a reduction in area of 34.5%, whereas the same steel alloy treated by the conventional quench and temper method developed a tensile strength of only 247,000 pounds per sq. in. and 0.7% reduction in area. It will be seen that high ductility is obtainable through the use of the present invention without sacrifice of high tensile strength.

In Fig. 2 we have set forth the thermal hardening time curve for a typical commercial steel alloy comprised of 0.78% carbon, 0.36% manganese, 0.160% silicon, 0.036% phosphorous, 0.048% sulphur and the balance substantially iron. In the Fig. 2, time is plotted on a logarithmic scale rather than on a linear scale in order to condense the curve to representable proportions. This type of curve is well known in the art and can be readily interpreted by those skilled in the art and is resorted to when necessary to illustrate great disparities in time intervals under the conditions portrayed by the chart.

From this chart it will be noted that the time interval for complete conversion of austenite to pearlite at 1300 F. comprises approximately'seven minutes; the time for complete conversion of austenite to fine pearlite at 1000 F. comprises approximately four seconds: the time for complete conversion of austenite to troostite at 600 F. comprises approximately ten minutes; the time for complete conversion of austenite to its decomposition product at 400 F. comprises approximately twenty-four hours and the time for complete conversion of austenite to fully hard martensite at 100 F. is of the order of one or two seconds.

In Fig. 3 we have indicated the hardness oblie tained on six common steels when hardened at various constant temperatures in accordance with the present invention in which curve B represents the steel of Fig. 2. It will be noted from curve B that a Rockwell hardness of about C 15 is obtained by causing this steel to transform completely at a temperature of 1300 F. Similarly a hardness of about C 36 results when the transformation from austenite takes place at 1000 F., a hardness of about C 51 when the transformation takes place at 600 F., a hardness of about C 57 when transformation takes place at 400 F. and a full martensitic hardness of about C 64 when the transformation takes place at F. or below.

From the curves shown in Figs. 2 and 3 it is believed clear that where a certain hardness, for example Rockwell C 51, is desired in a steel product of the steel composition'of Fig. 2, the same may be obtained by quenching the steel to a temperature approximating 600 F. and maintaining it at that temperature for about ten minutes.

.The steel product thus obtained will have an impact strength materially higher than the impact strength of a similar steel hardened to the same degree by prior art quench and temper methods. This is illustrated with particularity in Fig. 4.

In Fig. 4 curve (1) shows. the impact strength of a steel as obtained by the quench and temper method. Curve (2) shows the impact strength of the same steel as obtained by the practice of the present invention. The steel composition of each is substantially identical to that of the steel of Fig. 2.

Referring to Fig. 4 where a hardness of Rockwell C 51 is obtained the impact strength of a quench and tempered steel approximates 3 m4 foot pounds with a rod diameter of .180 inch. A similar steel rod hardened by the practice of the present invention to the same hardness (Rockwell C 51) has an impact strength approximating 35 foot pounds.

While we have disclosed as a specific embodiment of the practice of the present invention, the adaptation of the same to the hardening of wire, rope, cable or strip, it is not to be construed that we are limited thereby, as the specific invention is subject to many adaptations and modifications in the heat treatment of other steel products such as rails and shapes, cutlery, machine parts, etc. and such adaptations and modifications of the steps of the present thermal hardening method are contemplated as may be included within the scope of the accompanying claims.

What we claim is:

1. The method of thermally hardening carbon and allow steels which comprises quenching the same from atemperature above the critical temperature range to a temperature below the temperature of rapid pearlite formation but above that at which rapid conversion of austenite to martensite is obtained, and in thereafter maintaining the quenched steel at a temperature-above the temperatura of rapid martensite formation and below the temperature of rapid pearlite formation for a time interval suflicient to obtain substantial conversion of the austenite to crystal structures other than pure martensite characteristic for the temperature of heat treatment and then cooling. to atmospheric temperatures.

2. The method of thermally hardening carbon and alloy steels which comprises quenching theperature range to a temperature below about 1000 F. but above 300 F. and in thereafter maintaining the steel at a temperature within that range for a time interval suflicient to obtain substantial conversion of the austenite to crystal structures other than pure martensite characteristic for the temperature of heat treatment and then cooling to atmospheric temperatures.

3. The method of thermally hardening carbon and alloy steels which comprises quenching the steel from a temperature above the critical temperature range of the alloy to a temperature below approximately 1000 F. but above the temperature of rapid martensite formation and maintaining the steel at such an intermediate temperature for a time interval suflicient to obtain substantial conversion of the austenite to crystal structures other than pure martensite characteristic for the temperature of heat treatment and then cooling to atmospheric temperatures.

4. The method of thermally hardening carbon and alloy steels which comprises quenching the steel from a temperature above the critical temperature range of the alloy to a temperature below approximately 1000 F. but above the temperature of rapid martensite formation, holding the steel at such intermediate temperature for a time interval sufiicient to obtain substantial conversion of the austenite to crystal structures other than pure marensite characteristic for the temperature of heat treatment and thereafter quenching the steel to temperatures below about 300 F.

5. The method of thermally hardening steel wire, rope, cable, strip and the like which comprises heating the wire to temperatures above the critical temperature range, quenching the wire to temperatures above about 300 F. but below about 1000 F., holding the wire at that temperature for a predetermined desired time interval sufficient to obtain substantial conversion of the austenite to crystal structures other than pure martensite characteristic for the temperature of heat treatment and then cooling to atmospheric temperatures.

6. The method of thermally hardening steel wire, rope, cable, strip and the like which comprises heating the wire to temperatures above the critical temperature range, quenching the wire to temperatures above about 300 F. but below about 1000 F., holding the wire at that quench temperature for a predetermined desired time interval sufiicient to obtain substantial conversion of the austenite to crystal structures other than pure martensite characteristic for the temperature of heat treatment and thereafter quenching to room temperatures.

'7. The method of thermally hardening steel wire, rope, cable, strip and the like which comprises'heating the wire in a reducing atmosphere to temperatures above the critical temperature range, quenching the wire to temperatures above about 300 F. but below about 1000 F., holding the wire at that quench temperature for a predetermined desired time interval suificient to obtain substantial conversion of the austenite to crystal structures other than pure martensite characteristic for the temperature of heat treatment and then cooling to atmospheric temperatures.

8. The method of thermally hardening steel wire, rope, cable, strip and the like which comprises heating the wire in an inert atmosphere to temperatures above the critical temperature range, quenching the wire to temperatures above 10. As an article of manufacture, steel having the composition and hardened by the method, substantially as defined in claim 4.

11. As an article of manufacture, steel having the composition and hardened by the method substantially as defined in claim 1, said steel being characterized by being substantially free from martensite and by high ductility and impact strength.

EDGAR C. BAIN. EDMUND S. DAVENPORT. 

